This application claims the priority of Germany Patent Application No. 197 43 960.8, filed Oct. 4, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a brake system for motor vehicles having a brake pedal, a brake booster, a master brake cylinder and an electronically regulatable brake unit which is arranged between the master brake cylinder and the wheel brakes. An electronic control unit adjusts the brake pressure in the wheel brakes independently of the preliminary pressure existing at the output of the master brake cylinder.
A brake system of this type is known, for example, from German Patent document DE 195 24 939 A1. In the case of this known brake system, in the event of panic braking in dangerous situations, by means of the electronically regulatable brake unit, brake pressure is applied to the wheel brakes in the sense of a drastic braking beyond the actual brake pressure defined by the driver which is triggered by operating the brake pedal. In order to recognize panic braking, the preliminary pressure occurring at the outputs of the master brake cylinders, which is defined by the driver, is analyzed. If the rate of change of this preliminary pressure exceeds a defined threshold value, a drastic braking procedure is implemented. Such a process is also called "braking assist." In the case of such a known brake system, independently of the pedal force applied by the driver by way of the brake pedal, the electronically regulatable brake unit becomes active only when panic braking is recognized and drastic braking must therefore be carried out. The booster ratio of the brake booster, as the result of which, as a function of the pedal force applied by the driver by way of the brake pedal, a specific vehicle deceleration course occurs, is not considered in this case. In particular, in the case of this known brake system, no arbitrary rise of the deceleration gradient is provided in the absolute sense when the pedal force is increased.
From German Patent document DE 195 34 728 A1, a special further development of a brake booster is known by which, when a defined threshold value of the brake pedal force is reached, a rise of the deceleration gradient is achieved in the sense of an increase of the brake booster ratio. On the one hand, this known brake booster arrangement requires high mechanical expenditures and, on the other hand, it is inflexible with respect to the defined threshold value of the brake pedal force and the defining of a raised deceleration gradient.
It is an object of the invention to improve upon a brake system of the above-mentioned type such that an arbitrary deceleration rise in the event of an increasing pedal force is possible in a simple and flexible manner.
This object is achieved by a brake system for motor vehicles having a brake pedal, a brake booster, a master brake cylinder and an electronically regulatable brake unit which is arranged between the master brake cylinder and the wheel brakes. An electronic control unit adjusts the brake pressure in the wheel brakes independently of the preliminary pressure existing at the output of the master brake cylinder. The electronic control unit senses the course of a pedal operating value which is achieved by the brake booster and which is directly proportional to the pedal force applied by the driver by way of the brake pedal. When a predetermined threshold value of the pedal operating value has been reached, by means of the electronically regulatable brake unit, the brake pressure in the wheel brakes is generated such that, in the course of the deceleration, a raised deceleration gradient is reached in the case of a further rise of the pedal operating value in the sense of an increase of the brake booster ratio.
Alternatively or additionally, the electronic control unit senses the deceleration course achieved by the brake booster as a function of a pedal operating value which is directly proportional to the pedal force applied by the driver via the brake pedal. By means of the electronically regulatable brake unit, the brake pressure in the wheel brakes is generated such that a desired deceleration course corresponding to a desired characteristic deceleration curve stored in the control unit occurs when the actual deceleration course deviates from the desired deceleration course preferably within a defined tolerance band.
Advantageous further developments of the invention are described herein.
In a supplementary fashion, it is pointed out that the electronically regulatable brake unit preferably is a hydraulic unit, as illustrated, for example, on Page 212 of Automobiltechnische Zeitschrift ATZ, 1997, as part of the "DSC-System" by BMW. Preferably, the pedal operating value, which is directly proportional to the pedal force applied by the driver by way of the brake pedal, is formed by the preliminary pressure which is normally detected using a preliminary pressure sensor provided at the output of the master brake cylinder. The actual deceleration or the actual deceleration course can be determined, for example, by means of the brake pressure existing in the wheel brakes which is proportional to the vehicle deceleration or by means of the rotational wheel speed sensors, which exist in ABS systems anyhow, or by way of longitudinal acceleration sensors.
In a supplementary fashion, it is pointed out that the deceleration course achieved only by the brake booster, as a function of the pedal force applied by way of the brake pedal, is negatively affected, for example, by a vacuum which is too low, by a low friction value of the lining, by a coupling trailer and/or by a high payload. Also in the presence of these negative influential factors, the brake system according to the invention ensures a satisfactory vehicle deceleration for the driver.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.